Event ID - 100

Event Id100
Sourcemsftpsvc .
DescriptionThe server was unable to logon the Windows NT account anonymous@ftp.microsoft.com due to the following error: <error description> The data is the error code. For additional information specific to this message please visit the Microsoft Online Support site located at: http://www.microsoft.com/contentredirect.asp.
Event InformationCAUSE: This behavior can occur because the Users group does not by default have the Log on Locally user right, which is required to connect to resources hosted on an IIS server.
RESOLUTION: To resolve this issue, modify the Group Policy Object (GPO) for the domain controller to assign the Log on Locally user right to the Users group:
Click Start, point to Programs, point to Administrative Tools, and then click Domain Controller Security Policy.
Double-click Security Settings, double-click Local Policies, and then click User Rights Assignment.
Under Policy, click Log on Locally, and then click Add.
Click Browse, locate and click the appropriate group, and then click Add.
Click OK in the open dialog boxes to close them and save the new settings.
To refresh the group policy, start a command prompt, type the following command and press ENTER, and then press ENTER again when you are prompted:
secedit /refreshpolicy machine_policy /enforce
For more causes and resolution for this problem, refer the below given Microsoft article.

Following information from newsgroup post may help:
"Not ftp, or it would be from msftpsvc as the source. If your site is all anonymous, then check to see if your site is accessible from the internet. If your site is internal, and you have anonymous turned on, but you use basic or Integrated windows, then it appears your IUSR password is out of sync."
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"Experienced this issue after demoting a Win2K DC. Changed security user to local anonymous user (IWAN_XXX) and this enabled the anonymous login"
Reference LinksUsers Cannot FTP to a Windows 2000 IIS Server, System Reports "Access Denied" with Event ID 100

Users Cannot Access FTP or Web Site

Cannot Connect to FTP Site Anonymously When FTP Service Is Installed Alone

How to configure FTP folders and permissions for domain authentication in IIS

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